1. Field of the Invention
The present invention relates to a particle counter for measuring the number of floating particles contained in a sample to determine the particle concentration therein.
2. Description of the Prior Art
So-called false counts, which are displayed as a discrete value even when no particles of measurable size exist in a sample, are obtained in a particle counter. Noise generated by a laser beam source, noise generated by a photoelectric converter, noise resulting from random voltage variation of each circuit, cosmic rays invading from the outside and the like are cited as factors for causing the false counts.
In order to reduce the noise generated by the laser beam source, a laser diode is known which is driven by a laser drive circuit which outputs a drive electric current superimposing a high-frequency component on a direct electric current to make a longitudinal mode of the laser diode a multimode (e.g., Patent Document 1).
Patent Document 1: Japanese Patent Application Publication No. 9-178645
However, according to the particle counter described in Patent Document 1, the false counts resulting from the laser diode can be reduced, but there are still problems in that the false counts resulting from other factors can be untreatable.